Modality-Agnostic Hybrid Federated Learning via Knowledge Distillation and Reinforcement Learning-Based Aggregation

Due to multidimensional heterogeneity, Multimodal Federated Learning (MMFL) confronts fundamental challenges including modality incongruence, modality agnosticism, and modality incompleteness. Existing methods face a trilemma: leveraging external data with privacy risks, isolating features to restrict cross-modal interaction, or incurring high overhead from complex graph-based coordination, all culminating in suboptimal performance. In this paper, we propose a Modality-Agnostic Hybrid Federated Learning (MA-HyFL) framework that synergistically integrates unimodal and multimodal federated processes in modality-agnostic scenarios. Specifically, a bidirectional cross-modal knowledge distillation is employed to promote comprehensive collaboration at inter-client and intra-client levels, enabling robust knowledge transfer among heterogeneous modalities. A reinforcement learning-based aggregation mechanism is further introduced to orchestrate federated workflows through reward-driven policy optimization, dynamically integrating contributive client selection and adaptive aggregation weighting for closed-loop decision-making. Extensive experiments show that MA-HyFL significantly outperforms the other baseline methods in four realistic real-world applications, each exhibiting varying degrees of statistical heterogeneity and missing rates.